-LRB- CNN -RRB- -- The worst kept secret in Formula One is finally out -- Fernando Alonso is leaving Ferrari and will be replaced by Sebastian Vettel .

Red Bull 's four-time world champion has signed a three-year contract with the Scuderia , the oldest team in F1 , from 2015 .

After ending his five-year stint at Ferrari , Alonso remains coy on where he will be driving next season .

The double move by two of the sport 's high profile world champions is the most significant in the driver market this season .

But in the fickle world of F1 there are no guarantees it will work out for either of the ambitious racers .

There are still seats to be filled at McLaren , Force India and Toro Rosso . With the curtain about to fall on the 2014 season at Sunday 's Abu Dhabi Grand Prix , hopeful drivers have just one more chance to stake their claim for the remaining seats .

Where will Alonso go ?

Alonso is regarded as the best all-round driver currently racing at the elite level of motorsport , a fact many of his peers are even happily willing to acknowledge .

The Spaniard -- a double world champion with Renault in 2005 and 2006 -- is regarded as the key to the F1 driver market .

The 33-year-old made it clear he wanted to leave Ferrari , even though he had two years left to run on his contract with the Italian team , but he has yet to confirm where he will go next .

A return to McLaren seems most likely , despite his acrimonious departure from the team after just a single season as Lewis Hamilton 's teammate in 2007 .

McLaren is about to embark on a new phase after reigniting its relationship with engine manufacturer Honda .

Alonso is expected to lead this new era at McLaren , although the fiercely ambitious driver may first want guarantees that the Honda engine is going to be a success .

Keeping the media guessing about his future -- and maybe his future employers too -- seems to have provided Alonso with plenty of sport off track .

Speculation in the media has seen Alonso linked with buying the Lotus team , joining forces with German sports car specialists Audi and even ousting Nico Rosberg or Hamilton at Mercedes .

Alonso remained typically tight lipped about his future plans when he was questioned by the media at the Abu Dhabi season finale .

Has Vettel made the right decision ?

Vettel has endured his worst season at Red Bull in 2014 , failing to win a race compared to three victories for his rookie teammate Daniel Ricciardo .

Since his full debut season for Toro Rosso in 2008 , the German has won at least one race a year , not to mention winning four straight world championships between 2010 and 2013 .

The 27-year-old is now hoping to succeed where Alonso failed at Ferrari by adding to his collection of world titles .

`` The next stage of my Formula One career will be spent with Ferrari and for me that means the dream of a lifetime has come true , '' Vettel said .

`` When I was a kid , Michael Schumacher in the red car was my greatest idol and now it 's an incredible honor to finally get the chance to drive a Ferrari .

`` I am extremely motivated to help the team get back to the top . I will put my heart and soul into making it happen . ''

Vettel will partner Kimi Raikkonen -- the last man to win a world title with Ferrari in 2007 -- in 2015 but there are no guarantees the car and its Ferrari engine will be any match for the might of Mercedes .

Who will drive for McLaren ?

McLaren has tried to dampen speculation over just who will be in its cars in 2015 in the build-up to this weekend 's season-ending race .

`` We know you 're awaiting news on our driver line-up . We 'll announce after December 1 -- you 'll hear it here first , '' the team said on Twitter .

Alonso remains the red-hot favorite to take one of the seats but the future of current drivers Jenson Button and Kevin Magnussen remains unclear .

Button -- the 2009 world champion with Brawn Grand Prix which has since morphed into Mercedes -- has remained sanguine about his future and has even explored the idea of moving to sports car racing .

Danish rookie Magnussen is fiercely passionate about staying with McLaren , the team which gave the 22-year-old his F1 debut in 2014 .

`` There is no Plan B , '' the Dane told CNN when asked if had explored his options if he was n't retained by the eight-time world champions .

Which other teams have seats to fill ?

Mercedes , Red Bull , Ferrari , Williams , Lotus and Sauber have all confirmed their driver pairings for 2015 . There are still seats on offer at McLaren , Force India and Toro Rosso .

Force India has already announced it will retain German Nico Hulkenberg for 2015 but Mexican Sergio Perez has not had his position with the team confirmed .

There 's also only one seat up for grabs at Toro Rosso , who will blood 17-year-old Max Verstappen as the youngest F1 driver in history next season .

Junior Red Bull driver Carlos Sainz Jr is in pole position for the second seat -- and the Spaniard has been named as a test driver for Red Bull at the end of season test in Abu Dhabi -- although Toro Rosso may still retain French racer Jean-Eric Vergne .

Sauber announced Swede Marcus Ericsson and Brazilian Felipe Nasr as its 2015 drivers in November much to the chagrin of current driver Adrian Sutil , who believed he had a contract with the team for next season .

Ericsson and Nasr are both pay drivers , backed by sponsors who bring an estimated $ 30m funding to the Swiss team .

Grid shrinks to 18 in 2015 ?

The F1 market may be flooded with plenty of eager racers but the number of seats has been squeezed .

The financial pressures on the Caterham and Marussia teams , which both went into administration in October , means there are , in theory , only 18 spots on the F1 grid in 2015 .

After missing the U.S. and Brazil grands prix , Caterham has used crowdfunding to help finance a return in Abu Dhabi .

Japan 's Kamui Kobayashi retained his drive while the team has also handed Briton and former tester Will Stevens his F1 debut .

Both Caterham and Marussia remain on the official entry list for the 2015 season but their future is dependent on a hard winter drumming up funding to stay in the sport .

With teams spending a minimum of $ 70m per season , employing a driver who offers pace , performance and a pot of personal talent is more important than ever for those teams for ca n't afford super-talents like Alonso .

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Sebastian Vettel signs a three-year contract with Ferrari from 2015

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Red Bull 's four-time world champion replaces Fernando Alonso

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Alonso has still to announce which team he will drive for next season

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McLaren says it will not announce its driver line-up until 1 December